Capacitive sensor, the associated evaluation circuit and actuator for a motor vehicle

ABSTRACT

The invention relates to a capacitive sensor, the associated evaluation circuit and an actuator for a motor vehicle having the above-mentioned sensor for detecting the distance, speed, or position of an object, having a reference capacitance and two measuring capacitances, wherein a square-wave voltage is applied to the reference capacitance and the measuring capacitances via a resistor and a pulse which has a variable period and represents a measure of the particular measuring capacitance is obtained with the aid of a switching stage.

CROSS-REFERENCE TO RELATED APPLICATION

This application is the U.S. National Stage filing under 35 U.S.C. 371of international application number PCT/EP2015/069101, filed on Aug. 20,2015, the entire disclosure of each of which is incorporated herein byreference.

BACKGROUND OF THE INVENTION

The invention relates to a capacitive sensor, an associated evaluationcircuit and an actuator for a motor vehicle according to the preamblesof claims 3, 1 and 4.

Capacitive sensors are widespread not only in automation technology, butmore recently also in the automotive industry, where they findapplication, inter alia, as a trunk or door opening or seat occupancyrecognition.

DE 10 2012 106 526 A1 discloses a capacitive door handle sensor for amotor vehicle comprising at least two electrodes with differentmonitoring areas, wherein an electrode acts as a reference electrode(reference capacitance).

DE 10 2012 224 007 A1 discloses an arrangement and a method forconverting the capacity of a measuring capacitor into a voltage which isdetectable by an analog-digital converter comprising a charge transferdevice for transferring the charge of an unknown capacitance C_(x) to ameasuring capacitor C_(L), wherein, however, only the comparison of asingle unknown capacitance C_(x) with a reference capacitance C_(ref) isprovided.

WO 2007 025 785 A1 discloses an evaluation circuit for capacitivesensors, wherein two capacitors are compared with each other. For thispurpose two RC elements, each comprising one capacitor are supplied inparallel with the same square wave signal wherein their integrationtimes, which due to their capacities are different, are discriminated byuse of a XOR phase detector. The resultant square wave signal has apulse-to-pause ratio which is dependent on the measuring value and isintegrated by means of a further RC element so that an analog outputsignal is obtained. A disadvantage is that the circuit is not adaptedfor evaluating individual pulses but is only suitable for continuousoperation.

AT 403 213 B discloses a similar working capacitive soil moisture sensorin which a time constant which depends on the capacitance to be measuredresults in a phase distortion of a square wave signal. Here, as a phasedetector an AND gate is used, whose output signal is supplied to an RCfilter element. The RC filter element converts the square wave signalinto an analog signal which is dependent on its pulse-to-pause ratio. Adisadvantage is that only one unknown capacitance (sensor electrode) canbe measured at a time.

SUMMARY OF THE INVENTION

It is an object of the invention to provide an evaluation circuit for acapacitive sensor which produces a voltage which is detectable by meansof an analog-digital converter and is suitable for the evaluation of atleast two sensor electrodes.

Moreover, the evaluation circuit should be easy to calibrate and beimplementable with a minimum number of electronic components.

The object of the invention is achieved by the characterizing featuresof claim 1. The dependent claims relate to the advantageous embodimentof the invention.

The essential idea of the present invention is to compare the measuringcapacitances successively with the same reference capacitance in apredetermined time regime. To this end, the reference capacitance isconnected to the input of a first switching stage and the measuringcapacitances are connected to the inputs of further switching stages,wherein these further switching stages are configured, for example, asNAND gates and comprise enable inputs which are controlled by the outputof the first switching stage. The outputs of the further switchingstages are each connected to the input of an integration stage. Acharging capacitor is charged via the outputs of the integration stages,which may also act as current sources. The time length of the outputpulses generated from the switching stages determines the voltage of thecharging capacitor.

In an advantageous embodiment the reference capacitance is connected toa calibration unit which comprises at least a resistor, a capacitor anda voltage source or is connected to a controllable voltage source. Thecalibration unit is used both to improve the measurement accuracy bytargeted manipulation of the delay time generated by the referencecapacitance and to temporally optimize the measurement.

The advantage of the invention is that instead of at least twocalibration units only a single one is required. Thus, for example, acapacitive sensor with 3 sensor electrodes can be constructed by onlyone IC of the type 74HC132, which on the one hand leads to a saving ofcomponents and on the other hand improves the measurement accuracy.

The invention will be explained in more detail with reference to thedrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an evaluation circuit according to the invention comprising3 sensor electrodes; and

FIG. 2 shows the pulse diagram for controlling the measurement indetail.

DETAILED DESCRIPTION

FIG. 1 shows an exemplary embodiment comprising an integrated circuit ofthe type 74HC132 with a total of four switching stages 4 (NAND gates),wherein in the rest state, the controllable gate input of the switchingstage indicated with 41 is at the logic state “High” so that its outputassumes the logic state “Low”. This has the consequence that for theduration of this state the switching stages 42, 43, 44 have their inputswhich are not externally controllable at the logic state “Low”, too, andthus are signally locked, such that their outputs are at the logic state“High” for the duration of this signal state and the integration stages5 are also locked and the charging capacitor C_(a) indicated byreference symbol 8, which previously has been discharged via the switchS under the control of the abovementioned microcontroller μC, remainsdischarged.

The control input of the switching stage 41 which is not connected tothe operating voltage is connected to a calibration unit which inaddition to R_(ref) and C_(ref) has a calibration resistor R_(cal) and acapacitor C_(cal). In addition, two auxiliary power sources U₁ and U₂are connected via the capacitors C_(r1) and C_(r2).

In the rest state one of the clock inputs clock 1, clock 2, clock 3 isat “High” and the other two clock inputs are at “Low”. Thus, theswitching stage 42 whose clock input is “High” is prepared for pulsegeneration, while the other two clock inputs still remain locked bymeans of the logic input signal “Low” during this pulse generation.

In order to generate a pulse at one of the outputs of the switchingstages 42, 43, 44 by means of an externally connected control unit (notshown), e.g. a microcontroller (μC), both the clock input “clock 0” andthe clock input set at “High” of the clock inputs clock 1, clock 2,clock 3 are simultaneously switched to “Low”. Thus, the signal appliedto “clock 0” is supplied via the low pass R_(ref), C_(ref) to the inputof the respective NAND gate 41 and triggers a positive voltage jump atits output when reaching the threshold voltage, wherein the delay timeis influenced to a great extent by the calibration unit by means ofwhich the signal at C_(ref) can be shifted in time and be influenced inits course. To this end, the voltage U_(cal) and the auxiliary voltagesU₁ and U₂ are applied. These voltages and also the four shown clocksignals (clock 0, clock 1, clock 2, clock 3) may be generated by meansof the control unit (μC) described above.

For a reasonable pulse generation all time constants and all controlsignals which signally lie before the gate inputs of the gates 41 and42, 43, 44 are dimensioned or set such that at first the voltage at thegate input of the gate 41 reaches its negative switching threshold. Thiscauses the logic states at all gate inputs of the gates 42, 43, 44,which are inaccessible from the outside, to change from “Low” to “High”,such that, as shown, the gate 42 having its input which is accessiblefrom the outside set to the logic state “High” switches its output to“Low” and thus triggers an integration stage 5 connected downstream.Thus, a charging operation of the capacitor C_(a) designated with 8 isstarted via one of the integration stages 5. The other two gates 43 and44 in which the inputs accessible from the outside continue to be “Low”thus remain signally locked.

Thereafter, the voltage at the gate input accessible from the outsidewhose clock input is switched from “High” to “Low” simultaneously withthe signal “clock 0” reaches its negative switching threshold such thatthe gate output which has just been switched from “High” to “Low” isagain switched back to “High” and thus the triggering of the integrationstage 5 connected downstream is again interrupted such that the chargingoperation of the capacitor C_(a) designated with 8 is terminated.

Thus, upon reaching the threshold voltage of a first switching stage 41a start signal and upon reaching the threshold voltage of a secondswitching stage 42, 43, 44 a stop signal is generated.

The reference capacitance 1 is connected to a calibration unit 9 for atargeted modification of the delay time generated by the referencecapacitance 1, wherein the calibration unit 9 comprises at least aresistance R_(cal), a capacitor C_(cal) and a voltage source U_(cal)controllable by the evaluation unit (μC).

Thus, the duration of the triggering of the integration stages 5 isdependent on the electrode capacitance to be measured, which isassociated to the respective activated clock input (clock 1, clock 2,clock 3). For the evaluation of any capacitance to be measured therespectively associated clock input is triggered in the manner describedabove.

FIG. 2 shows the corresponding timing diagram in detail. The signalscorrespond to the clocks of FIG. 1. Here, the time-delayed clocks 2 and3 intended for the gates 43 and 44 not shown in FIG. 1 can be seen.

The time span indicated with “ts” is used to create a defined initialstate and must be at least as large as the sum of all delay times thatcould affect the electrical potential in the relevant capacitances andthus ensures a reproducible measurement sequence.

REFERENCE SYMBOLS

-   1 Reference capacitance,-   2 Measuring capacitance(s), measuring electrode(s)-   3 Square wave voltage, clock signal-   4 Switching stages (41, 42, 43, 44): NAND gate 74HC132 with Schmitt    Trigger-   5 Integration stage (bipolar Miller integrator or current source)-   6 Control signal for the gate inputs (42, 43, 44) inaccessible from    the outside-   7 Pulse duration (Δt) which represents the measured value-   8 Charging capacitor, integration capacitor-   9 Calibration unit consisting of R_(cal), C_(cal) and U_(cal) (for    all measuring electrodes)

The invention claimed is:
 1. Evaluation circuit for a capacitive sensorfor detecting the distance, speed or position of an object, comprising areference capacitance and two measuring capacitances, wherein thereference capacitance and the measuring capacitances are supplied with asquare wave voltage via a resistor, and with the aid of switching stagesa time variable pulse is obtained on each case which pulse is a measurefor the respective measuring capacitance, wherein the referencecapacitance is connected to the input of a first switching stage andeach of the measuring capacitances are connected to a respective inputof one of the further switching stages, wherein the further switchingstages include enable inputs which are triggered by the output of thefirst switching stage, wherein the outputs of the further switchingstages are respectively connected to the input of the integrationstages, wherein a charging capacitor is charged via the outputs of theintegration stages, wherein upon reaching the threshold voltage of thefirst switching stage a start signal and upon reaching the thresholdvoltage of the respective further switching stage a stop signal isgenerated.
 2. Evaluation circuit for a capacitive sensor according toclaim 1, characterized in that the reference capacitance is connected toa calibration unit for a targeted modification of the delay timeproduced by the reference capacitance, wherein the calibration unitcomprises at least a resistor, a capacitor and a voltage sourcecontrollable by the evaluation unit μC.
 3. Capacitive sensor comprisingan evaluation circuit for detecting the distance, speed or position ofan object, comprising a reference capacitance and two measuringcapacitances, wherein the reference capacitance and the measuringcapacitances are supplied with a square wave voltage via a resistor, andwith the aid of switching stage a time variable pulse is obtained oneach case which pulse is a measure for the respective measuringcapacitance, wherein the reference capacitance is connected to the inputof a first switching stage and each of the measuring capacitances areconnected to a respective input of one of the further switching stages,wherein the further switching stages include enable inputs which aretriggered by the output of the first switching stage, wherein theoutputs of the further switching stages are respectively connected tothe input of the integration stages, wherein a charging capacitor ischarged via the outputs of the integration stages, wherein upon reachingthe threshold voltage of the first switching stage a start signal andupon reaching the threshold voltage of the respective further switchingstage a stop signal is generated.
 4. Actuator in a motor vehiclecomprising a capacitive sensor comprising an evaluation circuit fordetecting the distance, speed or position of an object, comprising areference capacitance and two measuring capacitances, wherein thereference capacitance and the measuring capacitances are supplied with asquare wave voltage via a resistor, and with the aid of a switchingstages a time variable pulse is obtained one each case which pulse is ameasure for the respective measuring capacitance, wherein the referencecapacitance is connected to the input of a first switching stage andeach of the measuring capacitances are connected to a respective inputsof one of the further switching stages, wherein the further switchingstages include enable inputs which are triggered by the output of thefirst switching stage, wherein the outputs of the further switchingstages are respectively connected to the input of the integrationstages, wherein a charging capacitor is charged via the outputs of theintegration stages, wherein upon reaching the threshold voltage of a thefirst switching stage a start signal and upon reaching the thresholdvoltage of the respective further switching stage a stop signal isgenerated.
 5. Actuator for a motor vehicle according to claim 4 foractuating a door handle.